Nitrogen Dossier: Introduction

The different forms of nitrogen

The chemical element nitrogen (chemical symbol: N) occurs in several forms in our environment. N2, also known simply as ‘nitrogen’, is its best-known form. It is a colourless and odourless gas that occurs naturally: 78 percent of the Earth’s atmosphere is made up of N2. This form of nitrogen is beyond the scope of this nitrogen dossier because N2 remains in the air and does not contribute to nitrogen-based pollution.

In addition to this commonplace and relatively inert form (N2), nitrogen also occurs in more reactive forms, such as nitrogen oxides (NOx) and ammonia (NH3). In nature, these reactive forms of nitrogen only occur in small amounts, but when they are found in larger amounts, they lead to environmental problems. Nitrogen oxides (NO, NO2 and NO3) are emitted primarily by traffic (especially in exhaust fumes from diesel engine) and industry. Ammonia is released mainly by agriculture through emissions from manure (which represent 85 percent of ammonia emissions). Ammonia can also react to form ammonium (NH4+), an acid that has adverse effects on wildlife. Another form of nitrogen is nitrous oxide (N2O), although this is much less common (making up less than 1 percent of total nitrogen emissions). Again, agriculture is the main source of emissions. Nitrous oxide will not be considered further in this dossier because it represents such a small share of the nitrogen compounds in our environment. However, nitrous oxide is a greenhouse gas and is therefore included in our Greenhouse Gas Dossier.

The emission and deposition of nitrogen

By emission, we mean the release of molecules into the air, water or soil. In the case of nitrogen, this refers specifically to the emission of reactive forms, such as nitrogen oxides and ammonia, into the air. When nitrogen compounds are in the air in this form, they can end up on the ground through precipitation, a process referred to as deposition. The deposition of large amounts of reactive nitrogen compounds can lead to problems in natural areas. In nature, nitrogen compounds provide important nutrients for plants. However, the presence of excessive quantities of nitrogen compounds disrupts the natural balance: nitrogen-loving plants, such as nettles and grasses, grow faster and outcompete plants that thrive in environments with less nitrogen. In many natural areas in the Netherlands, the soil is naturally poor in nutrients, including nitrogen compounds. The deposition of extra nitrogen makes these areas more fertile, to the detriment of native species. A second problem associated with nitrogen deposition is soil acidification. This also causes certain types of plants to flourish at the expense of other species; for example, species of heather that are native to heathland can be overrun by grass. Such changes in local flora also have an impact on local wildlife.

Measuring the emission and deposition of nitrogen

Nitrogen emissions are usually expressed in kilograms of the various forms in which reactive nitrogen compounds (nitrogen oxides and ammonia) occur. However, when converting to kilograms of elemental nitrogen (N), it is important to remember that the molecular weights of the various forms of nitrogen vary. For example, one molecule of ammonia (NH3) is almost three times lighter than one molecule of nitrogen dioxide (NO2). This means that one kilogram of NH3 contains three times as much nitrogen (N) as one kilogram of NO2.

In order to account for the differences in the molecular weights of different nitrogen compounds, the decision was made to express nitrogen deposition in ‘moles of N/hectare’ so that the focus is on the number of nitrogen atoms. A mole provides an indication of the amount of nitrogen regardless of the form(s) in which that nitrogen occurs and the molecular weight of the compound concerned. The deposition of nitrogen from traffic, industry and agriculture leads to an increase in the concentration of nitrogen in soil and water. This concentration is usually expressed in micrograms of dissolved nitrogen (N) per cubic metre of air, soil or water.

The nitrogen balance in agriculture

In agriculture, nitrogen is supplied in various forms, including fertilisers and cattle feed (concentrate). Livestock excrete some of that nitrogen as manure, some of which dissipates into the air mainly as ammonia (emission), and some of which ends up in the soil or water. Some of the nitrogen is also converted into the form of proteins by the livestock, for example, and leaves the agricultural chain in the form of meat, milk and eggs. Crops such as cereals, vegetables and fruit also absorb nitrogen from manure or artificial fertilisers that are applied - thereby fixing the nitrogen for consumption. Some of the manure produced by the agriculture sector also leaves the sector through the export of manure or its use in amateur horticulture (e.g. vegetable gardens).

Based on the factors outlined above, it is possible to calculate the nitrogen balance for the agriculture sector as a whole. The nitrogen balance is equal to the amount of nitrogen supplied (inputs in the form of concentrates and fertilisers) minus the amount sequestered in animal and plant products and the use of manure outside agriculture (outputs). Essentially, the nitrogen balance refers to nitrogen inputs minus nitrogen outputs. A surplus means that excess nitrogen is escaping into water, soil and air (emissions). These variables are expressed in millions of kilograms of nitrogen.

Nitrogen in our environment since 1990

Even though they are interrelated, the various flows of nitrogen (such as emissions into the air, followed by deposition into water and soil) have followed different patterns over time. Most nitrogen flows have been decreasing since 1990, with the exception of nitrogen output from the agricultural sector. The ‘loss’ of nitrogen into the air and the nitrogen surplus have decreased significantly since 1990. These decreases are the result of the tightening of various regulations regarding manure, the aim of which was to reduce the nitrogen surplus. That downward trend now seems to come to an end for certain components of these nitrogen flows, however.

EU targets for reducing the deposition of nitrogen

Current legislation is based on two policy objectives set at the EU level in order to reduce nitrogen deposition: first, the critical deposition value for nitrogen in Natura 2000 areas (this value varies depending on the area and the type of habitat). In 2018, the deposition of nitrogen still exceeded the critical deposition value in most Natura 2000 areas. The second objective is the nitrogen ceiling for manure: since 2006, this has been set at a maximum of 504.4 million kg/N for the entire Dutch agricultural sector. Specific ceilings apply for particular types of livestock. In recent years, the Netherlands has often been (just) below the nitrogen ceiling, although the same is not always true of the ceilings for individual types of livestock.